Scroll wheel encoder
Also called scroll encoder
The encoder is the part that turns scroll wheel rotation into scroll steps. When a mechanical encoder gets dirty or worn the page jumps or scrolls briefly in the wrong direction.
In practice
Each notch you feel on a scroll wheel is the encoder clicking from one position to the next. Mechanical encoders use small contacts that wear and collect dust. The symptoms are pages that jump back a line while you scroll down, scrolling that skips steps or a wheel that does nothing for part of each turn.
How to check
Scroll slowly in one direction in the mouse test and watch the up and down counters. Every step should count in the direction you turned. A few counts in the opposite direction means the encoder is failing. Compressed air into the wheel gap sometimes helps. A replacement encoder is a common repair on many mice.